Ashcroft 100 PSI Selection Information

215

Product Selection Information

PRESSURE, TEMPERATURE AND
DIFFERENTIAL PRESSURE SWITCH

ELECTION

S

Before making your selection, consider the
following:

1. Actuator
T

he actuator responds to changes in pressure,
temperature or differential pressure and operates
the switch element in response to these changes.

The actuator is normally exposed to process fluid
and must therefore be chemically compatible
with it. The following may be used to help select
actuator type:

For nominal pressure ranges 0-15 psi through

-3000 psi, the standard actuator is a diaphragm-

0
sealed piston. In this actuator, process pressure
acting on the piston area causes it to overcome
the adjustment spring force and actuate a snap­action switch. A diaphragm and O-ring seal the
process media from this mechanism. These are
available in various materials, i.e.: Buna N, Teflon
and Viton. The standard process connection is
stainless steel. Optional monel pressure connec-

ion is available.

t

For ˝ H2O Pressure and Differential Pressure
Ranges, a diaphragm actuator is used. In this
design, the standard pressure connections are
carbon steel. Diaphragms are available in Viton,
Buna N and Teflon. Always review process tem-

erature limits before making seal selections.

p

Optional stainless steel pressure connections are
available (option XTA).

For High Differential Pressure Actuator Ranges,
3-15 to 60-600 psid, a Dual Diaphragm-Sealed
Piston Actuator is used. This actuator is designed
to for high static-pressure applications. The stan-

ard pressure connections are nickel-plated

d
brass. Diaphragms are available in Viton, Buna N
and Teflon. Always review process temperature
limits before making seal selections. Optional
stainless steel pressure connections are available
(option XUD).

For all temperature ranges the standard

®

Ashcroft

temperature actuator operates on the
vapor pressure principle: the vapor pressure in a
sealed thermal system is applied to a sensing
element, which in turn actuates a switch. This is
known as a SAMA Class II system. Various filling
materials are used, including Propane, Butane,
Methyl Alcohol, N Propyl Alcohol and Xylene.
High overtemperature capability is possible with
this type of system. The interface between liquid
and vapor is the point at which sensing occurs.
This is the “sensitive” portion of the bulb. Bulb
extensions and capillary are normally filled with
vapor, and have little effect on the setpoint,
regardless of ambient temperature variations;
therefore, no ambient compensation is required.
For best results, the bulb should be mounted
within 60 degrees of vertical to assure the liquid
remains in the bulb.

2. Enclosure

he enclosure protects the switch element and

T
mechanism from the environment and has provi-

ions for mounting and wiring. All Ashcroft

s
switch enclosures are epoxy-coated aluminum or
stainless steel for maximum corrosion resistance.
Choose between watertight NEMA 4, 4X for most
industrial applications and explosion-proof NEMA
7/9 for most process applications.

Ashcroft enclosures include watertight cover gas-

ets, external mounting holes and one or two

k

3

/4

NPT electrical conduit holes for ease of installa-

ion. Pressure switches may also be mounted

t
directly to the process by means of the standard

1

/4 NPTF or optional 1/2 NPT pressure connection.

Note: When installing Ashcroft switches, refer to
instruction sheets included with each switch, the
National Electrical Code, and any other local
codes or requirements to assure safety.

3. The Switching Function

ext, consider the switching function. Most appli-

N

ations for alarm and shutdown are satisfied by

c
single setpoint, fixed deadband models. For high/
low or alarm and shutdown, the dual setpoint
models may be selected. For pump, compressor,
level and other control applications, an adjustable
deadband model is often the best choice.

4. The Switch Element

inally, the electrical switching element must be

F
compatible with the electrical load being
switched. For ease of selection, all electrical

Consult factory for guidance in product selection
Phone [phone], Fax [phone] or
visit our web site at www.ashcroft.com

216

Product Selection Information

switching elements are snap acting, SPDT (single
pole-double throw), or 2 (SPDT). Select a switch
element with electrical rating that exceeds the
electrical rating of the device being controlled by
the switch. For better reliability and safety,
optional Hermetically Sealed switching elements
may be specified.

ADDITIONAL SWITCH TERMINOLOGY

Accuracy – (See repeatability) Accuracy normally

refers to conformity of an indicated value to an
accepted standard value. There is no indication in
switch products; thus, instead, the term repeat-

bility is used as the key performance measure.

a
Ashcroft switch accuracy is 1% of nominal
range.

utomatic Reset Switch – Switch which returns

A

to normal state when actuating variable (Pres­sure or Temperature) is reduced.

Adjustable or Operating Range – That part of
the nominal range over which the switch setpoint
may be adjusted. Normally about 15% to 100%
of the nominal range for pressure and differential
pressure switches and the full span for tempera-

ure switches.

t

Burst Pressure – The maximum pressure that
may be applied to a pressure switch without
causing leakage or rupture. This is normally at
least 400% of nominal range for Ashcroft
switches. Switches subjected to pressures above
the nominal range can be permanently damaged.

Deadband – The difference between the setpoint
and the reset point, normally expressed in units
of the actuating variable. Sometimes referred to
as differential.

Division 1 – A National Electrical Code Classifica-

ion of hazardous locations. In Division 1 loca-

t

ions, hazardous concentrations of flammable

t
gases or vapors exist continuously, intermittently
or periodically under normal conditions; fre-

uently because of repair or maintenance

q
operation/leakage or due to breakdown or faulty
operation of equipment or processes which
might also cause simultaneous failure of electri-

al equipment. Explosion-proof NEMA 7/9 enclo-

c

ures are required in Division 1 locations.

s

Division 2 – A National Electrical Code Classifica­tion of Hazardous locations. In Division 2 hazard-

us locations, flammable or volatile liquid or

o
flammable gases are handled, processed or
used, but will normally be confined within closed
containers or closed systems from which they
can escape only in case of accidental rupture or
breakdown or in case of abnormal operation of
equipment. Either Nema 7/9 explosion-proof
enclosures or any enclosure with hermetically
sealed switch contacts may be used in Division

locations.

2

Explosion Proof – A term commonly used in
industry referring to enclosures capable of with-

tanding an internal explosion of a specified gas

s
without igniting surrounding gases. Strict instal-

ation practices in accordance with the national

l
electrical code are also required for safety.

Fixed Deadband – The difference between the
setpoint and the reset point of a pressure or tem-

erature switch. It further signifies that this dead-

p

and is a fixed function of the pressure switch

b
and not adjustable.

Hermetically Sealed Switch – A switch element
whose contacts are completely sealed from the
environment to provide additional safety and reli-

bility. Contact arc cannot cause an explosion

a
and atmospheric corrosive elements cannot
affect the contacts.

Manual Reset Switch – Pressure or Tempera-

ure switch in which contacts remain actuated

t
even after the actuating variable returns to
normal. On Ashcroft manual reset switches, a
button must be pushed to reset the contacts.

National Electrical Manufacturers Association
(NEMA) – This group has defined several catego-

ies of enclosures, usually referred to as “types.”

r
Further, they designate certain features and capa-

ilities each type must include. For example,

b
among other features, a NEMA 4 enclosure must
include a threaded conduit connector, external
mounting provision and cover gaskets. When
selecting a NEMA 4 enclosure from any manu-

acturer, a buyer is assured of receiving these

f
features.

NEMA 4 – Watertight and dusttight enclosures
intended for use indoors or outdoors to pro­tect the equipment against splashing, falling
or hose-directed water
and water seepage. They are also sleet-resistant.

NEMA 4X – Watertight, dusttight and corrosion­resistant enclosures with same qualifications as
NEMA 4, but with added corrosion resistance.

NEMA 7 – Enclosures for indoor Class I, Division
1 hazardous locations with gas or vapor
atmospheres.

NEMA 9 – Enclosures for indoor Class II, Division
1 hazardous locations with combustible dust
atmospheres.

Normal Switch Position – Contact position
before actuating pressure (or variable) is applied.
Normally closed contacts open when the switch
is actuated. Normally open contacts close when
the switch is actuated.

Normally Closed – Refers to switch contacts
that are closed in the normal switch state or posi-

ion (unactuated). A pressure change opens the

t
contacts.

Normally Open Switch – Refers to the contacts
that are open in the normal switch state or posi-

ion (unactuated). A pressure change closes the

t
contacts.

, external condensation

Overpressure Rating(s) – A nonspecific term
that could refer to either burst or proof pressure,
or both.

Proof Pressure – The maximum pressure which
may be applied without causing damage. This is
determined under strict laboratory conditions
including controlled rate of change and tem-

erature: This value is for reference only. Consult

p
factory for applications where switch must oper-

te at pressures above nominal range or refer-

a

temperature (70°F).

ence

Repeatability (Accuracy) – The closeness of
agreement among a number of consecutive mea-

urements of the output setpoint for the same

s
value of the input under the same operating con-

itions, approaching from the same direction, for

d
full-range traverses. Ashcroft switch repeatabil-

is 1% of nominal range.

ity

Note: It is usually measured as nonrepeatabil-ity
and expressed as repeatability in percent of span
or nominal range. It does not include hysteresis
or deadband.

Reset Point – The reset point is the Pressure,
Temperature or Differential Pressure Value where
the electrical switch contacts will return to their
original or normal position after the switch has
activated.

Setpoint – The setpoint is the Pressure, Tem-

erature or Differential Pressure value at which

p
the electrical circuit of a switch will change state
or actuate. It should be specified either on
increase or decrease of that variable. (See also
reset point.)

Single-Pole Double Throw (SPDT) Switching
Element – A SPDT switching element has one

normally open, one normally closed, and one
common terminal. The switch can be wired with
the circuit either normally open (N/O) or normally
closed (N/C). SPDT is standard with most
Ashcroft pressure and temperature switches.

Snap Action – In switch terminology, snap action
generally refers to the action of contacts in the
switch element. These contacts open and close
quickly and snap closed with sufficient pressure
to firmly establish an electrical circuit. The term
distinguishes products from mercury bottle
types that were subject to vibration problems.

Static Pressure – For differential pressure
switches, static pressure refers to the lower of
the two pressures applied to the actuator.

Consult factory for guidance in product selection
Phone [phone], Fax [phone] or
visit our web site at www.ashcroft.com